The invention relates to an amplifier for electrical signals and which consists of a multiplicity of series-connected amplification stages of which each comprises at least one voltage-controlled amplifier element whose control characteristic can be adjusted by a control signal.
In a chain circuit of voltage-controlled amplification stages the problem occurs that the individual stages--with the same control voltage--can exhibit different amplifications due to production tolerances in the active elements, in particular in those which are equipped with semicondutors. This harbors the danger that during the amplification adjustment one of the first or front end amplification stages of the chain is over excited or over modulated earlier than the subsequent ones when strong input signals are present. The impulse prolongation causes thereby leads to fade-over dissolution of subsequent signals. However, this is generally not recognized as such; rather, the impulse widening is erroneously evaluated as a particularly strong echo. Correspondingly, the operator will further adjust to higher amplification degrees although overamplifications already occurred. The non-recognizance of this overamplification, however, then leads to falsifications of the measuring result. Different overamplifications or over excitations with a subsequent signal fade-over readily occur in such amplifiers, in which a control range of a correspondly broad dynamic response is required on the basis of the broad dynamic range of the incoming useful signals for the purpose of amplitude compensation. Signals with a particularly great dynamic range, for example, occur in the ultrasonic impulse echo-measurement technique. There the specific problem results that after irradiation of a transmitter impulse into an examination object, for example, a human body, in ultrasonic diagnostics, echo impulses of very different intensities occur from the various object layer depths. The larger the penetration depth, the smaller is the intensity of the incoming echo. The dynamic range of the echo signals can therefore extend between zero through 100 dB. It is obvious that with an over excitation or overmodulation of individual amplification stages by means of relatively strong echo impulses, subsequent echo impulses can no longer be detected and displayed. With B-scan, this leads, for example, to errors in the longitudinal resolution. The picture quality of sectional images is strongly decreased. The corresponding also holds true for A-scan or other ultrasonic scanning methods. In spite of reduced picture quality, allowances were made for the possible overamplification of individual amplification stages due to lack of suitable improvement suggestions, particularly because nobody thought of a numeric evaluation of the received echo signals until now. By numeric evaluation in the present case a processing method at the output signal can be generally understood. With the aid of this, conclusions can be drawn from the nature and form of the output signal about the respective properties of the original signal. In ultrasonic-diagnostics, for example, from the light intensity image occurring after the final amplification, conclusions can be drawn for the structure actually present in the examination object, which is only then possible when the properties of the transmission path from the object to the final evaluator are precisely known. Indeed, at least in regard to the disadvantages of over excitation, a sure remedy could have been provided if by active amplifier elements, a correspondingly stepwise higher-stage amplification would have been employed in the amplifier alongside of the chain with increasing amplification stage number. However, this method would have required a cumbersome selection from a production standpoint of active amplifier elements which would have unnecessarily increased the cost of the total equipment. Moreover, in an amplifier thus designed, the amplification of the individual stages would be necessarily different. Such an amplifier would thus be unsuitable for a numeric evaluation in the type described above.